HR: 1340h
AN: PP43B-0677    [Abstracts]
TI: Cenozo‹c History of the Equatorial Indian Ocean Recorded by Nd Isotopes: The Closure of the Indonesian Gateway
AU: * Gourlan, A T
EM: gourlan@ipgp.jussieu.fr
AF: Institut Physique du Globe de Paris, laboratoire de Geochimie-Cosmochimie 4 place jussieu, Paris, 75005 France
AU: Meynadier, L
EM: meynad@ipgp.jussieu.fr
AF: Institut Physique du Globe de Paris, laboratoire de Geochimie-Cosmochimie 4 place jussieu, Paris, 75005 France
AU: Allegre, C J
EM: allegre@ipgp.jussieu.fr
AF: Institut Physique du Globe de Paris, laboratoire de Geochimie-Cosmochimie 4 place jussieu, Paris, 75005 France
AB: The northward tectonic motion of the Australian plate and the evolution of the Indonesian Island Arcs through the last 20 Ma, generate changes in the flow and the origin of the circulation between the Pacific and the Southern Indian Oceans. Indeed, the emergence of the Indonesian Archipelago and probably the rapid uplift of the island of Halmahera have dramatically reduced the Indonesian Gateway. However, the precise dating of this event is still a matter of debate. The Neodymium isotopic composition of marine sediments is an extremely good proxy to reconstruct the major changes in the past ocean circulation. The residence time of Nd is shorter than the circulation time of the global ocean. Therefore, the Nd isotopic composition varies between the different ocean basins and is function of changes in source provenances, paleocirculation, orogenic processes, and intensity of weathering on the continents as well as on the volcanic arcs. To reconstruct the evolution of the oceanic flow from the Pacific to the equatorial Indian Ocean since the Miocene, we have applied on high carbonates content sediments a leaching technique using acetic acid. The reliability of our technique has been assessed by comparison with the Hydroxylamine hydrochloride technique developed by Bayon et al (1). The Nd isotopic composition is determinated in the past seawater from the record in Fe-Mn oxides. The sedimentary sequences are accurately dated using bio and chimiostratigraphy. Three ODP Sites were chosen in the Indian Ocean with a water depth ranging from 1600 to 2800 m and mutually distant by about 3000 km. From West to East: Site 761 which is at the western edge of the Indonesian Gateway on the central northeastern part of the Wombat Plateau off NW Australia, Site 757 is located on the south of the Ninetyeast ridge and Site 707 is located in the western tropical Indian Ocean near the Seychelles Islands. Our data are compared with the first results from Site 807 located in the Pacific Ocean on the northern rim of the Ontong Java Plateau. Results on the seawater signal of the Indian sites show a similar pattern in ε_Nd for the last 20 Ma with a continous increase in ε_Nd from -8 to -4.5 followed by an abrupt change of the ε_Nd values (decreasing at -6.5) which occurred around 3-4 Ma. Comparing our results with the Nd isotopic composition of three Fe-Mn crusts dragged close to the Indian sites (SS63, DODO 232D and VA16 13KD-1 (2,3)), we confirm a throughflow travelling west, with pacific waters entering across the Indonesian arcs, to the east of African coast since at least 15 Ma and support the idea of the rapid closure of the Indonesian seaway around 3-4 million years ago in less than 2 Ma. (1) Bayon et al, 2001, Chem. Geology 187,179 -199 (2) Franck et al, Chem. Geology 2005 submitted (3) O'Nions et al, 1998, EPSL, 155, 15-28
DE: 1000 GEOCHEMISTRY
DE: 1040 Radiogenic isotope geochemistry
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 4924 Geochemical tracers
DE: 8177 Tectonics and climatic interactions
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005